Children are normally screened at birth for those problems.
It is possible for children to have thyroid problems and this would certainly be worth looking at should all other tests come back normal. Your doctor may decide to do blood test for this anyway.
About.com on the link below talks about the symptoms to look out for in children suspected of having thyroid problems.
http://thyroid.about.com/b/2006/01/16/symptoms-of-thyroid-problems-in-children.htm
I do not think I have breached Medhelp policy because the site is not selling anything.
Best wishes.
I don't know how this can affect a small child, but if he were an adult, I would say get his thyroid checked. I think it's a good thing to do. He may have been born with a thyroid problem that was undetected. The thyroid is thermostat in your body. But, it's important that you get him checked out.
It could be that your little one is fighting a virus or an infection. It is very common for young children to be not well one minute and fine the next and this can go on for quite some time. Even if he appears fine, he may not be, so do get him checked out.
Book a doctor's appointment for him so that the doctor can examine him thoroughly. Unfortunately, young children can't always tell or pinpoint where the problem is.
Make sure that he has plenty of warm drinks so that he does not become dehydrated. If he is off his food, do not worry too much as long as he is getting in his fluids like water, diluted juices, soups. If he has no congestion and is OK with milk then that should be fine.
If you have a child thermometer, take readings of his temperature and write them down and the date and time that you took the reading, this will also help your doctor should you see him.
Best wishes.